1. Field of the Invention
The present invention concerns a high-capacity x-ray tube of the type having a rotating anode
2. Description of the Prior Art
High-capacity x-ray tubes as used, for example, in medical technology in CT systems occasionally are fashioned as rotating piston tubes, but mostly are fashioned as rotating anode tubes. In rotating piston tubes (such as, for example, specified in German OS 198 43 649) the anode is firmly connected with the rotatable tube piston (i.e., piston shaped vacuum housing), so the anode and cathode rotate together with the tube piston. In rotating anode tubes the cathode is fixed in the vacuum housing and the anode is rotatably arranged in the vacuum housing. By means of a suitable load-bearing part, the anode plate of the anode is attached to a bearing shaft that is rotatably mounted in the vacuum housing at bearings. The bearing shaft, and with it the anode plate, is rotated by means of an electro-actuator, for example by a squirrel-cage motor. The anode plate typically is composed of molybdenum with a coating of tungsten (W) or a W-alloy and a graphite plate soldered to it. Upon striking on the tungsten coating of the rotating anode plate, the electron beam originating from the stationary cathode generates an x-ray beam that can exit at an exit window in the vacuum housing. Such a rotating anode tube is described, for example, specified in German OS 100 17 777.
In newer versions, the load-bearing part that connects the anode plate with the bearing shaft is fashioned as a cylindrical or funnel-shaped component made of molybdenum. The attachment of anode plate to the load-bearing part ensues by a finely threaded screwed connection. After the assembly of these parts, the connection is soldered with a high-temperature solder, for example a zirconium solder. The molten solder reacts with the molybdenum of the components, resulting in an increase in the hardness of the solder. Tests have shown that the hardness values of the solder in the region of the load-bearing part drastically increase, in particular given higher capacity and therewith given stronger loading of the tube.
Examinations have furthermore shown that, during the operation of the x-ray tube to conduct scans of a subject, in each scan the outer region of the load-bearing part heats more significantly than the relatively cold location in the region of the solder connection. Given the design conditions of the aforementioned known versions, the load-bearing part (designated in the following as a “funnel”, due to the funnel-shaped embodiment) is under tension given rotation of the anode plate, with the highest tensile stresses occurring in the region of the solder connection. The plate edge heats substantially more significantly than the solder location, accordingly, the plate edge tends to expand more significantly than the solder. The solder location is thus the site with the highest tensile stress, which results from the temperature difference between the focal path ring of the anode plate and the colder (by comparison) solder location on the funnel.
Because the solder connection in the known embodiments represents a rigid material connection, the tensile stresses are fully transferred by the solder. Due to the relatively brittle character of the solder material, these tensile forces can lead to crack formations in the solder and to a flaking away (crumbling) of solder particles given severe loading of the tube. Incidentally connected with this is a reduction of the lifetime, and with it a premature failure of the tube.